A radiosensitizer is an agent used to enhance the effect of radiation therapy. Radiation therapy relies of two types of ionizing radiation: (1) subatomic particle radiation, which consists of alpha particles, beta particles (electrons), neutrons, protons, mesons, and heavy ions, and (2) electromagnetic radiation, which exists as a family of waves of varying frequency, including high-frequency x-rays. Electromagnetic radiation in the form of x-rays is most commonly used in megavoltage radiation therapy to treat common malignant tumors. Gamma rays, a form of electromagnetic radiation similar to x-rays but emitted by radioactive isotopes of radium, cobalt, and other elements, are also commonly used.
Subatomic particle radiation and electromagnetic radiation transfer energy to tissues as discrete packets of energy, called photons, that damage both malignant and normal tissues by producing ionization within cells. The target for these ions most commonly is the intranuclear DNA.
The damaging effects of radiation therapy are mediated by the radiation products of water: EQU H.sub.2 O.fwdarw.HO.sup..cndot. +H.sup..cndot. +e.sup.-.sub.aq +H.sup.+ +H.sub.2 +H.sub.2 O.sub.2
The hydroxyl radical, HO.sup..cndot., is an oxidizing radical and is primarily responsible for radiation damage. The radical is extremely reactive and short lived. It causes damage primarily in the vicinity in which it is generated and if it comes in contact with a solvated electron (e.sup.-.sub.aq), it will be neutralized.
The H.sup..cndot. radical is a reducing radical produced in low yield and reacts to a small extent but in a non-useful way. It, therefore, is not significant in the mechanism of radiation damage.
Solvated electrons, e.sup.-.sub.aq, are strong reducing radicals and highly energetic particles. They are very small by comparison to the hydroxyl radical and travel great distances quickly. They will neutralize hydroxyl radicals readily. Therefore, one of the mechanisms of a radiosensitizer is to "soak up" solvated electrons and prevent them from neutralizing hydroxyl radicals, thereby, allowing hydroxyl radicals to do their damage.
Most of the existing anticancer drugs act at the DNA level, either by direct chemical interaction, as do alkylating agents, or by interference with DNA biosynthesis, as in the case of antimetabolites (Daoud, S. S. et al., 1991). The major limitation with these drugs is their lack of selectivity, since the structure and biosynthesis of DNA in normal cells is only marginally different from that in tumor cells (Tritton, T. R. et al., 1985). As a result, most of the existing antineoplastic reagents have undesirable toxic effects on rapidly dividing normal cells. To some degree, the specificity for hypoxic cells can be improved by proper selection of sidechains. Several classes of reagents have been programmed to attack cellular DNA, including reagents which intercalate between base pairs in the duplex and those which bind to the phosphate layer by electrostatic association.
The development of reagents that act against cellular loci other than DNA may lead to drugs that provide effective antitumor treatment and which are less toxic than currently available radiation sensitizers. One of these cellular loci is the cell membrane (Kale, R. K. et al., 1990). Some degree of drug specificity at this site seems possible because numerous subtle differences in composition, structure, organization, dynamics, and function exist between the surfaces of normal and hypoxic cells which may be exploited to design selective reagents. Thus, drugs which severely perturb membrane function or cause lipid peroxidation represent an important class of reagents. These systems are based on the formation of peroxyl radicals under in situ radiolysis in the presence of low concentrations of oxygen.
The biological responses to radiation-induced cell injury are modulated by various endogenous and exogenous radioprotectors (Painter, R. B., Radiation Biology in Cancer Research, pp. 59-68, 1980). Sulfhydryl compounds, including cysteine, cysteamine, and dithiothreitol, have been shown to protect living cells against the lethal effects of ionizing radiation. It has also been observed that depletion of cellular sulfhydryl compounds can result in radiosensitization (Biaglow, J. E. et al., 1983). Two main mechanisms may be inferred for the action of sulfhydryl substrates. One involves scavenging of water radiolysis products, notably hydroxyl radicals. Evidence has also accumulated to show that sulfhydryl compounds can repair radiation-induced damage to DNA by hydrogen atom donation to transient oxidized species (Cadet, J. et al., 1988). Thus, an important feature of drug design revolves around attempting to block the repair mechanism.
Cells may circumvent the lethal effects of toxins by altering the levels of proteins involved in their metabolic activation. Reduction in the levels of enzymes required for conversion of some reagents into their toxic metabolites has been shown to be an effective mode of drug resistance, particularly when the parent compound is relatively nontoxic (Townsend, A. J. et al., 1989). Enzymes may also play a key role in resistance by enabling cells to convert toxins into less active or more easily removed chemical species. Although some of these detoxification pathways have been well characterized for many environmental and industrial toxins, their role in cellular resistance to antineoplastic drugs has only recently been appreciated. Among the strikingly diverse set of enzymes found in mammalian cells, glutathione S-transferases seem to be especially effective for removing anticancer drugs from the system (Townsend, A. J. et al., 1989). Depletion of this enzyme could have a significant effect on the performance of certain drugs.
It is now recognized that cytotoxicity may result from minor changes in cellular levels of calcium or protons and from inhibition of gene expression. Such processes, if combined with selective uptake of reagent into infected cells, offer great potential for radiotherapy because they require minimal generation of the active agent. This is in contrast to lipid peroxidation which has to be widespread before effective necrosis is observed.
Hypoxic cells in solid tumors are relatively resistant to cell killing by radiation as well as by conventional chemotherapeutic drugs (Grau, C. et al., 1992). Although various types of electron-affinic reagents are known to promote radiosensitization of cells with diminished oxygen supply, relatively few show activity at nontoxic doses with in vivo models. Clinical trials with one of the better known reagents, misoimidazole, have revealed some therapeutic gain but the compound cannot be used at optimal dose due to neurotoxicity (Pan, S. -S. et al., 1990). Approaches aimed at improving the therapeutic performance of nitroimidazoles have included lowering the lipophilicity so as to restrict nervous tissue penetration and accelerate renal clearance and modifying the electron affinity. The objective has been to promote in situ formation of a stable .pi.-radical anion that can react with cellular oxygen to form superoxide ions.
Hypoxia in solid tumors is known to result in a decreased effectiveness of ionizing radiation and hypoxic cells distant from blood vessels may also be resistant to drug-based therapy because of insufficient drug delivery, reduced drug activation under hypoxic and/or acidic conditions, or low activity against non-cycling hypoxic cells. On the other hand, certain drugs may be selectively toxic to hypoxic cells and might to used to improve treatment in radiotherapy. Information about the specific effects of reagents on well oxygenated and hypoxic cells is necessary for the development of rational chemotherapeutic regimens designed to attack each of the physiological tumor subpopulations. Thus, extensive in vitro testing of new reagents must be made under both aerobic and hypoxic conditions and promising reagents must be tested against established standards. The most common standards are cyclophosphamide, 5-fluorouracil, and misoimidazole. It has been reported (Grau, C. et al., 1988) that whereas 5-fluorouracil is more effective against well oxygenated cells, misoimidazole and cyclophosphamide are toxic towards both oxic and hypoxic cells under radiolysis.
The present invention provides new compounds for radiosensitization. Texaphyrins enhance radiation damage and the enhancement is unaffected by the present of oxygen. Texaphyrins and water soluble texaphyrins have been described in U.S. Pat. Nos. 4,935,498 and 5,162,509 and US applications Ser. Nos. 07/822,964, now U.S. Pat. No. 5,252,720 and 08/075,123, now abandoned, all of which are incorporated by reference herein.